Cellular telephone networks were originally developed to provide primarily voice services over circuit-switched networks. Although circuit-switched networks are still in widespread use, the current trend is toward packet switched networks that provide not only voice services, but also high-speed packet data services, that enable mobile users to surf the web, read email, download video and audio files, and do many other things that Internet users can do on fixed networks. However, real-time multimedia remains a problem in mobile networks because most packet data services are provided by best effort networks that lack quality of service guarantees needed for real-time multimedia services. Also, due to lack of standardization, network operators are often limited to offering only those IP services that their equipment vendors support. Lack of standardization also makes it difficult for network operators to purchase bundled services from third party developers.
The IP multimedia subsystem (IMS) was developed to provide a common, standardized architecture and standardized interfaces for providing IP services in a mobile networking environment. The IMS network is not dependent on the access technology and will interoperate with virtually any packet-switched network, including UMTS, cdma2000, GPRS and EDGE networks. IMS uses the session initiation protocol (SIP) as the service control protocol, which allows operators to offer multiple applications simultaneously. The IMS standard will speed the adoption of IP services on mobile terminals, allowing users to communicate via voice, video, or text using a single client on the mobile terminal.
Although IMS promises a richer experience to mobile subscribers, network operators are hesitant to invest in equipment to implement IMS until there are a sufficient number of subscribers with IMS capability to make the investment worthwhile. Most cellular telephones currently in use lack IMS capabilities, so the pool of potential subscribers for IMS services is relatively small. Extending IMS capabilities to legacy mobile terminals that lack inherent IMS capabilities would provide a much broader market for network operators and encourage investment in IMS technology and equipment.